\documentclass{article}


%宏包
\usepackage{amsmath}
\usepackage{graphicx}
\newtheorem{thm}{Theorem}[section]
%正文
\begin{document}


\title{linex loss function}
\author{}
\date{}
\maketitle
%\tableofcontents
%\mainmatter
%\part{}
%\chapter{}


\section{model}
hello model
$ f(x) = \sum_{i=1}^{n} 2^i $
\section{test}
hello test！



\end{document}